For the measurement of temperatures, especially with electrical or electronic instruments, various basic principles have been employed. For example, it is known to measure temperatures by detecting the potential difference or emf produced by a junction of dissimilar metallic and/or semi-conductive temperature-sensitive elements such as thermocouples or thermopiles. Such systems produce an emf which is a function of temperature and may be used for a wide variety of temperature-measuring purposes.
However, when such thermoelements are employed for the measurement of temperatures above about 1000.degree.C or when the measuring instruments or sensor is to be located in an environment which may be detrimental, the accuracy of the measurement leaves much to be desired and considerable error is introduced.
Apparently maintaining the dissimilar-metal junction at an elevated temperature for long periods varies the emf per .degree.C which is generated by the system, perhaps as a result of interdiffusion of the metals, diffusion of impurities from a furnace atmosphere into the junction or like changes in the sensor. The prolonged exposure to high temperatures may also affect the leads or conductors and these disadvantages are observed, even when the system is enclosed in a ceramic sleeve.
As described in the above-identified applications, it has been proposed to avoid the disadvantages of conventional temperature-sensing systems by providing so-called noise thermometers which utilize a different principle. A noise-thermometer system utilizes a metallic strand, wire or film which generates an electrical output by thermal agitation of electrical changes within the conductor. The output is a noise voltage and is produced in the electrical conductor by such thermal agitation. Thermal noise, also known as JOHNSON noise, can be produced in a conductor even at temperatures approaching 0.degree.K at which thermocouples become noticeably less efficient, and may be particularly suitable for the measurement of temperatures in the range of several hundred .degree.K. The available thermal-noise power is proportional to the absolute temperature over the frequency band width over which the noise is measured, and described in the aforementioned applications. With a fixed band width, the available available thermal-noise power can be measured in terms of the noise voltage and is proportional to absolute temperature. The theory of such systems and various circuits utilizing the principles of JOHNSON noise and temperature measurement, are described in U.S. Pat. Nos. 2,710,899, 2,728,835, 2,768,266 and 2,884,786.
Frequently it is desirable to have available another temperature-measuring instrument with which a thermocouple can be calibrated with the aid of a noise thermometer or other temperature detector. For this reason two instruments are required and the introduction of both simultaneously to the measurement site may pose a problem. Furthermore, when reference to one and another indicator must be made repeatedly, the problem has been all but insurmountable with conventional systems.